The present invention relates to light quantity adjusting devices provided in still cameras, video cameras, or other image pickup apparatus that open and close blades, such as shutter blades or diaphragm blades, to adjust the quantity of photographing light.
In general, these light quantity adjusting devices adjust the quantity of light for use by a shutter or a diaphragm by operation of an electromagnetic driver to open and close a blade located on a photographing optical axis of a camera or the like. The electromagnetic driver is composed of a magnet rotor having a permanent magnet and a rotating shaft provided in the center of the permanent magnet, and a stator having a coil frame and an exciting coil wound around the outer periphery of the coil frame. The electromagnetic driver energizes the exciting coil to rotate the magnet rotor by a predetermined angle to open and close the blade.
There has been a strong demand for a reduction of sizes of these electromagnetic driving light quantity adjusting devices. For example, for cameras incorporated into small-sized instruments such as cellular phones, light quantity adjusting devices such as shutter blades or diaphragm blades are required to have a drastically reduced size and reduced power consumption. In a driving section of the light quantity adjusting device, the rotating shaft of the magnet rotor is rotatably borne by the coil frame, around which the exciting coil is wound and which is externally covered by a sleeve-like yoke. The yoke has had its diameter reduced to about 2 to 3 mm and a further reduction in yoke diameter has been demanded.
A problem in reducing the size of the driving section is how to bear the rotating shaft of the magnetic rotor (hereinafter referred to as a “rotor shaft”) by means of a frame such as the coil frame. A bearing structure is known such that the rotating shaft is borne by fitting an end of the cylindrical shaft into a cylindrical bearing hole. In such a bearing structure, for example, an end surface of the magnet rotor is slidably contacted with the frame to support the rotor shaft in the thrust direction in order to prevent the rotor shaft from moving in a thrust direction (axial direction). An outer peripheral wall of the rotor shaft is slidably contacted in a radial direction with an inner wall of the bearing hole. This results in a heavy frictional load on the bearing.
Unfortunately, a reduction in the shaft diameter of the rotor shaft may also lead to frequent jolts of the rotor shaft in the bearing hole. It is difficult to machine the components so as to form an appropriate gap in the fitting portion.
Thus, for example, Japanese Patent Laid-Open No. 2004-138939 proposes the use of a pivot bearing structure that bears the rotor shaft in the coil frame. Specifically, Japanese Patent Laid-Open No. 2004-138939 proposes a bearing structure having a magnet rotor shaft, a coil frame, a coil, and a yoke annually arranged on a rear surface of a substrate in this order, with a blade located on the substrate. In this structure, a lower end of the rotor shaft is formed to be spherical or conical and supported by a conical or spherical bearing hole.
With such a pivot bearing, when the opposite ends of the rotating shaft (rotor shaft) are pointed and supported by V groove bearings, the rotating shaft may jolt up and down unless a space is very accurately formed between V groove bearings, located at the respective ends of the rotating shaft. Thus, according to the Japanese Patent Laid-Open No. 2004-138939, one end of the rotor shaft is pointed and supported by the V groove bearing. The other end is formed to be cylindrical and supported by the cylindrical bearing. The rotor shaft is magnetically urged against the V groove bearing so that loads act on the V groove. This stabilizes the rotor shaft by reducing the load acting on the rotor shaft.
For example, Japanese Patent Laid-Open No. 2004-138939 discloses that a pivot bearing, undergoing relatively light loads in the thrust direction, be employed when the rotor shaft is stably supported with the bearing load reduced as described above. Thus, in the known bearing structure, as shown in FIGS. 8(a) and 8(b), a coil frame 51 around which a coil is wound is mounted on a substrate 50. A V or U-shaped bearing groove 52 is formed at a lower end of the coil frame 51 to support a pointed shaft end 53a formed on a rotor shaft 53. The other end of the rotor shaft 53 is formed into a cylindrical shaft end 53b, the outer periphery of which is supported by a cylindrical bearing 54. An arm 55 is integrally attached to a cylindrical shaft end 53b supported by the cylindrical bearing 54. The arm 55 is engaged with a blade 56 mounted on the substrate 50.
In the above conventional bearing structure, the rotor shaft 53 is positioned by the illustrated bearing groove 52 and may be tilted around this point (pivot point). This is because a clearance needs to be formed between the cylindrical shaft end 53b, located at the upper end of the rotor shaft 53, and the cylindrical bearing 54, which fittingly supports the periphery of the cylindrical shaft end 53b, and may vary owing to machining accuracy. That is, the frictional load is increased when the outer diameter d1 of the cylindrical shaft end 53b is equal to the inner diameter d2 of the cylindrical bearing 54 (d1=d2). A large dimensional difference between the outer diameter d1 and the inner diameter d2 increases the gap to tilt the rotor shaft 53 as shown in the figure.
Such a clearance is required for smooth rotations, and it is considered to be almost impossible to maintain the machining accuracy necessary for a uniform clearance. For example, when machining is performed so that the rotor shaft has a circularity and an outer diameter accurate to at least 1/100 mm, costs and the number of failures in a manufacturing process increase. This also applies to machining of the cylindrical bearing.
Then, when the lower end of the rotor shaft is pivotally supported and the arm is provided at the upper end to drivingly open and close the blade, as in the prior art, the following problem may occur. The rotor shaft 53 is tilted (through an illustrated angle α) when a clearance (gap) larger than required is formed between the cylindrical shaft end 53b at the upper end of the rotor shaft 53 and the cylindrical bearing 54, which supports the cylindrical shaft end 53b. This inclination (angle α) results in a displacement Y in the blade 56, which is thus jolted. The displacement Y=2Ly·sin(βy/2)·cos(α+βy/2). The magnitude of jolt increases in proportion to the length Ly between a pivot point o1 and the engaging point o2, between the arm 55 and the blade 56.
When a jolt occurs between the rotor shaft and the blade, if the blade performs, for example, a shutter operation, then disadvantageously, an optical path aperture cannot be fully closed or opened. Further, if the blade performs a restricting operation, then disadvantageously, the quantity of light cannot be adjusted to a value meeting photographing conditions or hunting may occur during operation to vibrate the blade, which becomes uncontrollable making the apparatus defective.
Thus, an object of the present invention is to provide a light quantity adjusting device that reduces frictional loads on a rotor shaft supported by bearings to allow a blade to be stably driven and to further minimize jolting of the blade.
Another object of the present invention is to provide a simple, inexpensive light quantity adjusting device that has a reduced size and reduced power consumption.
Further objects and advantages of the invention will be apparent from the following description of the invention.